This invention relates to the field now well established of ansa-metallocenes which are used as catalysts. They are particularly useful as catalysts for the polymerization of .alpha.-olefins.
Conventional heterogeneous catalysts such as Ziegler-Natta systems have a variety of active sites, only some of which are stereo-specific. Obtaining a polymer with specific properties can involve a considerable amount of trial and error in order to find the best combination of catalyst, co-catalyst and stereo-regulator. In contrast, however, the active polymerization site in a metallocene catalyst is well defined, and can be modified in a straightforward manner via modification of the cyclopentadienyl ligands, enabling the structure of the polymer to be controlled with far greater precision.
A simple metallocene catalyst for polymerizing ethylene (C.sub.5 H.sub.5).sub.2 ZrCl.sub.2 which consists of a zirconium atom bound to two chlorine atoms and two cyclopentadienyl rings, and which is activated by co-catalysts such as methylaluminoxane (MAO). During the 1980's, ansa, or bridged, metallocenes, in which the cyclopentadienyl rings are linked by a chemical bridge, were found to be particularly useful for the polymerization of olefins. In particular, ansa-metallocene complexes, when used in combination with a co-catalyst such as methylaluminoxane (MAO), polymerize propylene to highly isotactic polypropylene, highly syndiotactic polypropylene, or atactic polypropylene, depending on the structure of the ansa-metallocene used.
As is well known, isotactic polymers have each pendant group attached to the backbone in the same orientation, whereas in syndiotactic polymers, these groups alternate in their orientations and atactic polymers have a random arrangement of the groups along the backbone. Since the stereochemistry of the polymer has a great effect on its properties, it is desirable to control this feature. Chiral, C.sub.2 -symmetric ansa-metallocenes produce isotactic polypropylene.
Chiral rac ansa-zirconocene complexes, when activated by MAO or other cocatalysts, are excellent catalysts for isotactic .alpha.-olefin polymerization and other stereoselective reactions. Brintzinger, H. H.; Fischer, D.; Mulhaupt, R.; Rieger, B.; Waymouth, R. M., Angew. Chem., Int. Ed. Engl., 1995, 34, 1143. Hoveyda, A. H.; Morken, J. P. Angew. Chem., Int. Ed. Engl., 1996, 35, 1262. Racemic SiMe.sub.2 -bridged bis-indenyl zirconocenes that contain methyl and aryl substituents at the indenyl 2 and 4 positions, respectively, are among the best metallocene catalysts for the production of high molecular weight, isotactic poly(.alpha.-olefins). Spaleck, W.; Kuber, F.; Winter, A.; Rohrmann, J.; Bachmann, B.; Antberg, M.; Dolle, V.; Paulus, E. F., Organonmetallics, 1994, 13, 954. Stehling, U.; Diebold, J.; Kirsten, R.; Roll, W.; Brintzinger, H. H.; Jungling, S.; Mulhaupt, R.; Langhauser, F., Organometallics, 1994, 12, 964.
While the greatest area of potential use for ansa-metallocene catalysts currently is for polymerization of olefins, such as ethylene and propylene, they also have significant uses as catalysts or catalyst precursors for other reactions where stereo-selectivity is important.
The utility of ansa-metallocene complexes as catalysts for olefin polymerization and other reactions has created a high demand for practical syntheses of ansa-metallocene compounds.
In spite of this demand, current procedures for the synthesis of Group IV (Ti, Zr, Hf) ansa-metallocenes are hampered by low yields and tedious isomer separation and purification steps. Some of these problems have been discussed in Ellis, W. W.; Hollis, T. K.; Odenkirk, W.; Whelan, J.; Ostrander, R.; Rheingold, AS. L.; Bosnich, B. Organometallics 1993, 12, 4391. In particular, the synthesis of chiral C.sub.2 symmetric ansa-metallocenes typically produces mixtures of desired racemic (rac) and undesired meso isomers and separation of the rac from the meso products is not always possible.
Ansa-zirconocenes are normally synthesized by salt-elimination reactions between bis-indenyl anion reagents and ZrX.sub.4 or ZrX.sub.4 L.sub.2 compounds (L=Lewis base). However, the factors that control chemoselectivity (i.e., metallocene vs. dinuclear products) and diastereoselectivity (i.e., rac/meso selectivity) in these reactions are not well understood, and extensive screening studies of reagents, counterions, solvents, use of added ligands, and reaction conditions are required for each case to optimize yields. Strickler, J. R.; Power, J. M, U.S. Pat. No. 5,847,175, issued 1998. Rohrmann, J.; Kuber, F., U.S. Pat. No. 5,616,747, issued 1997. Fischer, D.; Schweier, G.; Brintzinger, H. H.; Damrau, H. R. H., Eur. Pat. Applic. 0 745 606 A2, 1996.
Ansa-metallocenes can also be prepared by amine elimination reactions of ansa-bis cyclopentadienes and metal amide complexes as discussed in U.S. Pat. No. 5,597,935. However, the amine elimination reaction produces amines such as NMe.sub.2 H as byproducts which must be disposed of. Additionally, the amine elimination approach does not work well with some titanium or hafnium amides, crowded metal amides or crowded or weakly acidic cyclopentadienes. Some limitations of this method are described in Diamond, G. M.; Jordan, R. F., Petersen, J. L. Organometallics 1996, 15, 4030 and Christopher, J. N.; Jordan, R. F.; Petersen, J. L.; Young, V. G. Jr. Organometallics 1997, 16, 3044.
There is, therefore, a need for a process which would produce ansa-metallocene complexes, especially zirconocene complexes, in high yield. Additionally, there is a need for a process which will produce rac ansa-metallocenes in high yield without contamination by the meso isomer, since the rac isomer is most useful in stereoselective catalysis. The present invention fulfills these needs. These and other objectives, features, and advantages will become apparent after review of the following description and claims of the invention.